1. Field of the Invention
The invention relates generally to downhole seismic sources and, more particularly, but not by way of limitation, it relates to an improved type of orbital vibration source with an epicyclic eccentric weight system for generating both shear wave and compression wave energy into an earth medium.
2. Description of the Background Art
Many different types of downhole seismic source mechanisms have been utilized for inducing acoustic energy into a borehole wall and subsequently analyzing velocity and/or phase of received energy to ascertain certain lithologic information. There are currently ongoing efforts in the industry to develop suitable downhole sources for generation of shear waves and/or compression waves with greater power and high signal-to-noise ratios to enable new approaches to seismic data acquisition. Current efforts have taken various forms and utilize diverse mechanisms in attempts to impart a clean, concise seismic energy input through the borehole wall or, in some cases, the well casing, and it is contemplated that development of such sources will open new possibilities of seismic data acquisition and interpretation.
U.S. Pat. No. 4,874,061 issued Oct. 17, 1989 to Cole, entitled "Downhole Orbital Seismic Source" relates to a downhole seismic source for the generation of elliptically polarized seismic shear waves and compression waves in a borehole. The invention uses the sonde casing of a generally cylindrical shape which houses an interactive rotational disk or the like to impart transverse rotational motion through the coupling wellbore fluid into the borehole wall. The invention contemplates a number of different types of basic orbital motion generators including motor driven eccentric disks.
U.S Pat. No. 4,709,362 issued Nov. 24, 1987 to Cole entitled "Oscillating Orbital Vibrator" relates to an orbital vibratory mass used for generating seismic signals and the like. The orbital vibrator comprises a hollow cylindrical reaction mass which is mounted by means of a bearing on a crank shaft. The reaction mass orbitally oscillates relative to the axis of the crank shaft without rotating. The eccentricity of the crank causes the reaction mass to orbitally oscillate about the center line of the shaft in response to the shaft's rotation. Since the reaction mass orbitally oscillates instead of rotating, this reduces the internal forces created in the reaction mass by centrifugal force and enables less exotic, less expensive materials to be used in its construction.
Co-pending U.S. patent application No. 07/954,831 entitled "Downhole Hydraulic Rotary Seismic Source" filed Sep. 29, 1992, relates to an improved type of orbital vibration source with a hydraulic power means for generating both shear wave and compression wave energy into an earth medium. The orbital source features a bearing mounted rotor which is optimized for weight eccentricity to total rotor weight and with minimized rotational wind drag. The source provides distributed force contact between the internal components and the external housing and allows two hydraulic motors with single shafts synchronously to drive two separate rotor assemblies which are located axially such that a tendency to excite a fundamental bending mode is minimized. The invention includes a built in lubricant recirculating system intended to lengthen the life of the rotor bearings.